Some Adelaide bus operators are to have their payments reduced over failures to meet timetable obligations.

Transport Services Minister Chloe Fox will make an announcement in coming days and says she is getting legal advice on one contractor which has defended its performance between January and March.

Ms Fox says there has been some ongoing underperformance by some bus contractors.

"Last time the fee reductions that they received were fairly much, you know, I'd call them moderate," she said.

"Obviously that might not necessarily be the case this week.

"We recognise that there are issues with congestion, we recognise that there are issues with movement through the city, all those things but these companies need to remember that they are operating on timetables that they bid to deliver."

Almost $40,000 of fines were paid by public transport bus companies last year for poor performance.

"There has to be even further incentive for them to get it right," the Minister said.

The Opposition said fines issued in the past had been weak, but Ms Fox said that was not the view of the bus contractors.

"None of these companies were particularly happy with the very moderate fines they got the last time and I can tell you that if you'd been a fly on the wall during some of those post-fine conversations you would have been fairly astonished by the vehemence of their views about what they were fined," she said.

Ms Fox said the next round of timetable changes would be made in July.